The invention relates to a method for operating a sensor for determining the concentration of oxidizing gases, especially for determining the nitrogen oxide concentration in exhaust gases of internal combustion engines.
Such a sensor is presented, for example, in U.S. Pat. No. 5,942,190.
Electrical fields and electrical currents arise between the individual electrodes and between the electrodes and the heater which cause the measuring result to be incorrect. This occurs because all electrodes of such a sensor are conductively is connected to the solid state electrolyte and the insulation layer of the heater has a finite resistance and, accordingly, all electrodes are connected to each other via electrically conductive structures and are connected at high resistance to the heater.
The method of the invention offers the advantage that the measuring errors can be eliminated by active compensation or at least can be minimized. The measurement errors arise because of the mutual coupling of the electrodes via electrical fields and currents in the solid state electrolyte as well as by the voltage drops across the feed line resistances. It is possible to precisely adjust the voltages on the electrodes without voltages on the electrodes being made incorrect by erroneous voltage drops on the electrode feed lines. This is made possible by changing the voltages, which are applied to the electrodes in accordance with function, in dependence upon the currents which flow in the electrode feed lines and/or between the electrodes. It is especially advantageous that the adjustment is independent of the current intensity with which the individual electrodes are charged.
An advantageous embodiment provides that one adds voltages to the voltages applied to the electrodes. The voltages added correspond to a feedback of voltage components weighted with factors and these voltage components are proportional to the currents. Furthermore, the sliding mean values of the voltages and/or their derivation of higher order and/or their sliding mean values or linear combinations thereof can be fed back. These voltages are proportional to the currents and the mean values are formed by means of known electric circuit elements. In this way, it is also possible to eliminate capacitive couplings.
The adjustment of the voltage on the electrodes takes place in this case advantageously by changing these factors. These factors are increased until the system starts to oscillate because of the feedback. The oscillation arises when the fed back factor is xe2x89xa71 in magnitude and, at the same time, the phase is greater than or equal to 180xc2x0. Then, the factors are reduced slightly but only so far that just no oscillation occurs anymore. In this way, almost all voltage drops, which arise at the electrode feed lines, as well as the voltage drops which arise because of a fictive resistance network within the solid state electrolytes, can be compensated.